Pros
You get a portable for work. Good benefits and start salary. Some people are very brilliant there. Transfer possibilities. Open to remote work (depends...) Interesting client sites. Being on the bench often I got good at interviews. Landed my dream job at Google.
Cons
Bench experience. Slow and too much management. No innovation. CGI talks and writes cute stuff. They are doers, but not achievers. You are definitely a number within this company. You have to be "politically correct". You have to play the game and don't be honest. Bad culture. Legacy technology. Terrible HR tools. Cash cow. The whole principle of CGI is to hire cheap people and management get all the credit. Incompetence brings promotion. You want promotion, you might want to wait for a decade. No competency. Some old folk will say that things are not done this way even you give them all the good and obvious reason. Old mentaly in tech. Most people there stick to one thing all there life and they are confortable with it. Disgusting augmentation even if you are crucial to the team.